Video hologram and device for reconstructing video holograms

ABSTRACT

The invention relates to video holograms and devices for reconstructing video holograms, comprising an optical system having a light source, lens and the video hologram having cells arranged in a matrix or a regular pattern with at least one opening per cell, the phase or amplitude of said opening being controllable. The holographic video representations of expanded spatial objects can be achieved in a wide viewing area in real time using controllable displays, whereby the objects are either computer-generated or created by different means. The space-bandwidth product (SBP) of the hologram is thus reduced to a minimum and the periodicity interval of the Fourier spectrum is used as a viewing window on the inverse transformation plane, through which the object is visible in the preceding space. The mobility of the viewer(s) is achieved by tracking the viewing window.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/534,877,filed May 12, 2005, which is the U.S. national phase of InternationalApplication No. PCT/DE2003/003791, filed Nov. 11, 2003, which is basedon and claims priority to German Application No. DE 102 53 292.3, filedNov. 13, 2002, the entire contents of which are hereby incorporatedfully herein by reference.

BACKGROUND OF THE INVENTION

The claimed invention relates to a video hologram and a device forreconstructing video holograms comprising an optical system, thatconsists of at least one light source, a lens and a hologram-bearingmedium composed of cells arranged in a matrix or an otherwise regularpattern with at least one opening per cell, the phase or amplitude ofsaid opening being controllable, and a viewing plane located in theimage plane of the light source.

Devices for reconstructing video holograms using acousto-opticalmodulators (AOM) are known from prior art (Stephen A. Benton, Joel S.Kollin: Three dimensional display system, U.S. Pat. No. 5,172,251). Suchacousto-optical modulators transform electric signals into optical wavefronts, which are recomposed in a video frame using deflection mirrorsto form two-dimensional holographic areas. A scene visible for theviewer is reconstructed from the individual wave fronts using furtheroptical elements. The optical means used, such as lenses and deflectionelements, have the dimensions of the reconstructed scenes. Due to theirgreat depth, these elements are voluminous and heavy. It is difficult tominiaturise them, so that their range of applications is limited.

Another possibility to generate large video holograms is provided by theso-called “tiling method”, using computer-generated holograms (CGH). Inthis method, known from WO 00/75698 A1 and U.S. Pat. No. 6,437,919 B1,small CGHs having a small pitch are composed with the help of an opticalsystem. For this, in a first step, the required information is writtento fast matrices having a small pitch (usually EASLM [electronicallyaddressable spatial light modulators]), and then the matrices arereproduced on to a suitable holographic medium and composed to form alarge video hologram. Usually, an optically addressable spatial lightmodulator (OASLM) is used as holographic medium. In a second step, thecomposed video hologram is reconstructed with coherent light intransmission or reflection.

In the CGH with controllable openings arranged in a matrix or in anotherwise regular pattern, known e.g. from WO 01/95016 A1 or Fukaya etal., “Eye-position tracking type electro-holographic display usingliquid crystal devices”, Proceedings of EOS Topical Meeting onDiffractive Optics, 1997, the diffraction on small openings is takenadvantage of for encoding the scenes. The wave fronts emerging from theopenings converge in object points of the three-dimensional scene beforethey reach the viewer. The smaller the pitch, and thus the smaller theopenings in the CGHs, the greater is the diffraction angle, i.e. theviewing angle. Consequently, with these known methods enlarging theviewing angle means to improve the resolution.

As is generally known, in Fourier holograms the scene is reconstructedas a direct or inverse Fourier transform of the hologram in a plane.This reconstruction is continued periodically at a periodicity interval,the extension of said periodicity interval being inversely proportionalto the pitch in the hologram.

If the dimension of the reconstruction of the Fourier hologram exceedsthe periodicity interval, adjacent diffraction orders will overlap. Asthe resolution is gradually decreased, i.e. as the pitch of the openingsrises, the edges of the reconstruction will be distorted increasingly byoverlapping higher diffraction orders. The usable extent of thereconstruction is thus gradually limited.

If greater periodicity intervals and thus greater viewing angles are tobe achieved, the required pitch in the hologram comes closer to thewavelength of the light. Then, the CGHs must be sufficiently large inorder to be able to reconstruct large scenes. These two conditionsrequire a large CGH having a great number of openings. However, this iscurrently not feasible in the form of displays with controllableopenings (see EP 0992163 B1). CGH with controllable openings onlymeasure one to several inches, with the pitches still beingsubstantially greater than 1 μm.

The two parameters, pitch and hologram size, are characterised by theso-called space-bandwidth product (SBP) as the number of openings in thehologram. If the reconstruction of a CGH with controllable openings thathas a width of 50 cm is to be generated so that a viewer can see thescene at a distance of 1 m and in a 50-cm-wide horizontal viewingwindow, the SBP in horizontal direction is about 0.5*10⁶. Thiscorresponds to 500,000 openings at a distance of 1 μm in the CGH.Assuming an aspect ratio of 4:3, 375,000 openings are required in thevertical direction. Consequently, the CGH comprises 3.75*10¹¹ openings,if three colour sub-pixels are taken into consideration. This numberwill triplicate if the fact is taken into account that the CGH withcontrollable openings usually only allows the amplitudes to be affected.The phases are encoded taking advantage of the so-called detour phaseeffect, which requires at least three equidistant openings per samplingpoint. SLM having such a great number of controllable openings arehitherto unknown.

The hologram values must be calculated from the scenes to bereconstructed. Assuming a colour depth of 1 Byte for each of the threeprimary colours and a frame rate of 50 Hz, a CGH requires an informationflow rate of 50*10¹²=0.5*10¹⁴ Byte/s. Fourier transformations of dataflows of this magnitude exceed the capabilities of today's computers byfar and do thus not allow holograms to be calculated based on localcomputers. However, transmitting such an amount of data through datanetworks is presently unfeasible for normal users.

In order to reduce the enormous number of computations it has beenproposed not to calculate the entire hologram, but only such parts of itthat can be seen directly by the viewer, or such parts that change. Thekind of hologram which consists of addressable sub-regions, such as theabove-mentioned “tiling hologram”, is disclosed in the above-mentionedpatent specification WO 01/95016 A1. Starting point of the calculationsis a so-called effective exit pupil, the position of which can coincidewith the eye pupil of the viewer. The image is tracked as the viewerposition changes by continuous recalculation of the hologram part thatgenerates the image for the new viewer position. However, this partlynullifies the reduction in the number of computations.

The disadvantages of the known methods can be summarised as follows:Arrangements with acousto-optical modulators are too voluminous andcannot be reduced to dimensions known from state-of-the-art flatdisplays; video holograms generated using the tiling method aretwo-stage processes which require enormous technical efforts and whichcannot easily be reduced to desktop dimensions; and arrangements basedon SLM with controllable openings are too small to be able toreconstruct large scenes. There are currently no large controllable SLMwith extremely small pitches, which would be needed for this, and thistechnology is further limited by the computer performance and datanetwork bandwidth available today.

SUMMARY OF THE INVENTION

The invention is defined in the claims. In one implementation, videoholograms and devices for reconstructing video holograms withcontrollable openings according to the present invention arecharacterised in that in the viewing plane at least one viewing windowis formed in a periodicity interval as a direct or inverse Fouriertransform of the video hologram, said viewing window allowing a viewerto view a reconstruction of a three-dimensional scene. The maximalextent of the viewing window corresponds to the periodicity interval inthe plane of the inverse Fourier transformation in the image plane ofthe light source. A frustum stretches between the hologram and theviewing window, said frustum containing the entire three-dimensionalscene as Fresnel transform of the video hologram.

The viewing window is limited approximately to and positioned inrelation to one eye, an eye distance of a viewer or to another suitablearea.

In an implementation, another viewing window is provided for the othereye of the viewer. This is achieved by the fact that the observed lightsource is displaced or added a second, real or virtual, adequatelycoherent light source at another suitable position to form a pair oflight sources in the optical system. This arrangement allows thethree-dimensional scene to be seen with both eyes through two associatedviewing windows. The content of the video hologram can be changed, i.e.re-encoded, according to the eye position in synchronism with theactivation of the second viewing window. If several viewers view thescene, more viewing windows can be generated by turning on additionallight sources.

In another implementation of the device for reconstructing a videohologram, the optical system and the hologram-bearing medium arearranged so that the higher diffraction orders of the video hologramhave a zero point for the first viewing window or an intensity minimumat the position of the second viewing window. This prevents the viewingwindow for one eye to cross-talk the other eye of the viewer or to otherviewers. It is thus taken advantage of the decrease in intensity of thelight towards higher diffraction orders, which is due to the finitewidth of the openings of the hologram-bearing medium and/or the minimaof the intensity distribution. The intensity distribution forrectangular openings, for example, is a sinc² function which rapidlydecreases in amplitude and forms a sin² function which decreases as thedistance grows.

The number of openings in the display determines the maximum number ofvalues that must be calculated for the video hologram. The transmissionof data from a computer or through a network to the display representingthe video hologram is limited to the same number of values. The dataflow rate does not substantially differ from the data flow rates knownfrom typical displays used today. Now, this will be illustrated with thehelp of an example.

If the viewing window is reduced, for example, from 50 cm (horizontal)by 37.5 cm (vertical) to 1 cm by 1 cm by choosing a sufficientlylow-resolution display, the number of openings in the hologram will dropto 1/1875. The required bandwidth is reduced in the same way during datatransmission through a network. Video holograms created with knownmethods require 10¹² openings, while this number is reduced to 5*10⁸pixels in this example. The scene can be viewed in full through theremaining viewing window. These requirements on pitch and hologram sizeaccording to the space-bandwidth product can already be fulfilled bydisplays available today. This allows to inexpensively realise largereal-time video holograms on displays with large pitch for a largeviewing window.

The viewing window is tracked by mechanically or electronicallydisplacing the light sources, by using movable mirrors or by using lightsources which can be adequately positioned in any other way. The viewingwindows are displaced according to the displacement of the light sourceimages. If the viewer moves, the light source(s) is (are) spatiallydisplaced so that the viewing windows follow the eyes of the viewer(s).This is to ensure that the viewers can also see the reconstructedthree-dimensional scene when they move, so that their freedom ofmovement is not limited. Several systems are known for detecting theposition of the viewers, e.g. systems based on magnetic sensors can beused beneficially for this.

An implementation of this invention also allows to reconstruct a videohologram efficiently in colour. Here, the reconstruction is performedwith at least three openings per cell, representing the three primarycolours, amplitude or phase of said openings being controllable, andsaid openings being encoded individually for each of the primarycolours. Another possibility of reconstructing a video hologram incolour is to perform at least three reconstructions one after another,namely for the individual primary colours, using the device of thepresent invention.

An implementation of this invention allows to efficiently generateholographic reconstructions of spatially extended scenes throughcontrollable displays, such as TFT flat screens, in real-time andproviding large viewing angles. These video holograms can be usedbeneficially in TV, multimedia, game and design applications, in themedical and military sectors, and in many other areas of economy andsociety. The three-dimensional scenes can be generated by a computer orin any other way.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is illustrated and explainedbelow in conjunction with the accompanying drawings, wherein

FIG. 1 is a general illustration of a video hologram and a device forreconstructing video holograms showing the generation of the diffractionorders and the position of a viewing window;

FIG. 1 a shows the content of FIG. 1 with a magnified view of an exampleof a focusing lens system 2 comprising two single lenses;

FIG. 2 is a general illustration of a device for reconstructing videoholograms showing a three-dimensional scene which can be viewed througha viewing window;

FIG. 3 is a general illustration of a device for reconstructing videoholograms showing the encoding of the three-dimensional scene in a partof the video hologram;

FIG. 4 is a diagram showing the light intensity distribution in theviewing plane depending on the diffraction orders;

FIG. 5 is a general illustration of a device for reconstructing videoholograms showing the position of the viewing windows for both eyes of aviewer with regard to the diffraction orders to prevent cross-talking;

DETAILED DESCRIPTION

A device for reconstructing video holograms comprises thehologram-bearing medium, a sufficiently coherent, real or virtual, pointor line-shaped light source and an optical system. The videohologram-bearing medium itself consists of cells which are arranged in amatrix or in an otherwise regular pattern with at least one opening percell, the phase or amplitude of said opening being controllable. Theoptical system for reconstructing the video hologram can be realised byan optical imaging system known in the art, consisting of a point orline laser or a sufficiently coherent light source.

FIG. 1 shows the general arrangement of a video hologram and itsreconstruction. A light source 1, a focusing lens system 2, shown as asingle lens, for the purpose of simplicity, a hologram-bearing medium 3and a viewing plane 4 are arranged one after another, seen in thedirection of the propagating light. The viewing plane 4 corresponds withthe Fourier plane of the inverse transform of the video hologram withthe diffraction orders. FIG. 1A shows the content of FIG. 1 with amagnified view of an example of a focusing lens system 2 comprising twosingle lenses.

The light source 1 is imaged on to the viewing plane 4 through anoptical system, represented by the lens system 2. If a hologram-bearingmedium 3 is inserted, it (the hologram-bearing medium 3 being encodedwith a hologram) is reconstructed to comprise focal points (e.g., apoint 7 of a reconstructed three-dimensional scene 6 as shown in FIG. 3)before the viewing plane 4 (i.e., between the hologram-bearing medium 3and the viewing plane 4) and as an inverse Fourier transform in theviewing plane 4. The hologram-bearing medium 3 with periodic openingscreates equidistantly staggered diffraction orders in the viewing plane4, where the holographic encoding into higher diffraction orders takesplace, e.g. by way of the so-called detour phase effect. Because thelight intensity decreases towards higher diffraction orders, the 1^(st)or −1^(st) diffraction order is used as the viewing window 5. If notexplicitly expressed otherwise, the 1st diffraction order will be takenas a basis in the further description of the invention.

The dimension of the reconstruction was chosen here to correspond withthe dimension of the periodicity interval of the 1^(st) diffractionorder in the viewing plane 4. Consequently, higher diffraction ordersare attached without forming a gap, but also without overlapping.

Being the Fourier transform, the selected 1^(st) diffraction order formsthe reconstruction of the hologram-bearing medium 3. However, it doesnot represent the actual three-dimensional scene 6. It is only used asthe viewing window 5 through which the three-dimensional scene 6 can beobserved (see FIG. 2). The actual three-dimensional scene 6 is indicatedin the form of a circle inside the bundle of rays of the 1^(st)diffraction order. The scene is thus located inside the reconstructionfrustum which stretches between the hologram-bearing medium 3 and theviewing window 5. The scene 6 is rendered as the Fresnel transform ofthe hologram-bearing medium 3, whereas the viewing window 5 is a part ofthe Fourier transform.

FIG. 3 shows the corresponding holographic encoding. Thethree-dimensional scene is composed of discrete points. A pyramid withthe viewing window 5 being the base and the selected point 7 in thescene 6 being the peak, is prolonged through this point and projected onto the hologram-bearing medium 3. A projection area 8 is created in thehologram-bearing medium 3 that point being holographically encoded insaid projection area. The distances between the point 7 to the cells ofthe hologram-bearing medium 3 can be determined in order to calculatethe phase values. This reconstruction allows the size of the viewingwindow 5 to be constrained by the periodicity interval. If, however, thepoint 7 was encoded in the entire hologram-bearing medium 3, thereconstruction would extend beyond the periodicity interval. The viewingzones from adjacent diffraction orders would overlap, which would resultin the viewer seeing a periodic continuation of the point 7. Thecontours of a thus encoded surface would appear blurred due to multipleoverlapping.

The intensity decrease towards higher diffraction orders is takenadvantage of to suppress cross-talking to other viewing windows. FIG. 4shows schematically a light intensity distribution over the diffractionorders, said distribution being determined by the width of the openingsin the CGH. The abscissa shows the diffraction orders. The 1^(st)diffraction order represents the viewing window 5 for the left eye, i.e.the left viewing window, through which the three-dimensional scene canbe viewed. Cross-talking into a viewing window for the right eye issuppressed by the decrease in light intensity towards higher diffractionorders and, additionally, by the zero point of the intensitydistribution.

Of course, the viewer can view the scene 6 of the hologram 3 with botheyes (see FIG. 5). For the right eye, the right viewing window 5′represented by the −1^(st) diffraction order of the light source 1′ waschosen. As can be seen in the drawing, this light influences the lefteye at a very low intensity. Here, it corresponds to the −6^(th)diffraction order.

For the left eye, the 1^(st) diffraction order corresponding to theposition of the light source 1 was chosen. The left viewing window 5 isformed likewise. According to an implementation of this invention, thecorresponding three-dimensional scenes 6 and 6′ (not shown) arereconstructed using the light sources 1 and 1′ in a fix position inrelation to the eyes. For this, the hologram 3 will be re-encoded whenthe light sources 1 and 1′ are turned on. Alternatively, the two lightsources, 1 and 1′, can simultaneously reconstruct the hologram 3 in thetwo viewing windows 5 and 5′.

If the viewer moves, the light sources 1 and 1′ are tracked so that thetwo viewing windows 5 and 5′ remain localised on the eyes of the viewer.The same applies for movements in the normal direction, i.e.perpendicular to the video hologram.

Further, several viewers can view a three-dimensional scene ifadditional viewing windows are created by turning on additional lightsources.

1. A method for reconstructing a three-dimensional scene using areconstruction device including a light source, an optical system and ahologram encoded on a hologram-bearing medium having a matrix of cells;the hologram-bearing medium and the optical system being illuminated bythe light source; and the optical system imaging the light source intoan image plane of the light source; the method comprising the steps of:(i) the optical system generating an inverse Fourier transform of thehologram encoded on the hologram-bearing medium at the image plane ofthe light source; (ii) providing a viewing window at the image plane ofthe light source, the viewing window being the location where anobserver places at least one eye to view the holographic reconstructionrepresenting the three-dimensional scene; and (iii) forming theholographic reconstruction of the three-dimensional scene from thehologram encoded in the hologram bearing medium within a reconstructionfrustum stretching between the hologram-bearing medium and the viewingwindow.
 2. The method of claim 1, wherein the viewing window ispositioned in relation to an eye of an observer.
 3. The method of claim1 in which the holographic reconstruction of the three-dimensional sceneis made up of multiple discrete points and the hologram on thehologram-bearing medium comprises the limited region with informationneeded to reconstruct one such single point in the reconstruction, thepoint being visible from the viewing window, and is characterized inthat the limited region: (a) is encoded with information for that singlepoint in the reconstruction and (b) is the only limited region in thehologram encoded with information for that point, and (c) is restrictedin size to form a portion of the entire hologram, the size being suchthat multiple reconstructions of that point caused by higher diffractionorders are not visible at the viewing window.
 4. The method of claim 3in which the limited region has been generated by a projection from theviewing window through the single point onto the hologram-bearingmedium.
 5. The method of claim 1 comprising the step of timesequentially re-encoding a hologram on the hologram-bearing medium forone eye and then the other eye of an observer.
 6. The method of claim 1in which the holographic reconstruction representing thethree-dimensional scene is described by the Fresnel transform of thehologram being encoded in the hologram-bearing medium and theholographic reconstruction representing the three-dimensional scene isnot described by the Fourier transform of the hologram being encoded inthe hologram-bearing medium.
 7. The method of claim 1 in which the sizeof the viewing window is calculated by a computing means in dependenceof the extension of the periodicity interval of the hologram bearingmedium.
 8. The method of claim 1 in which the size of the viewing windowis smaller than the size of the hologram-bearing medium.
 9. The methodof claim 1 in which there are separate viewing windows, one for each eyeof the observer.
 10. The method of claim 9 in which each viewing windowis approximately 1 cm×1 cm.
 11. The method of claim 9 in which thelocations of an observer's eyes are tracked and the positions of theviewing windows are altered so that the observer can maintain a viewthrough each viewing window even when moving his or her head.
 12. Themethod of claim 1 in which the light source includes one or moreindividual light sources.
 13. The method of claim 1, in which the lightsource is a line-shaped light source.
 14. The method of claim 1, inwhich the light source is a real light source.
 15. The method of claim1, in which the light source is a point light source.
 16. The method ofclaim 1, wherein several light sources are turned on to generate viewingwindows for several observers.
 17. The method of claim 1, whereininformation required to determine the position of the light source isprovided by at least one position sensor that measures the position ofthe observer.
 18. The method of claim 1 comprising the steps ofassigning a first viewing window to a first eye of a viewer and alsoassigning a second viewing window to the other eye of the viewer, thesecond viewing window being generated using a second light source. 19.The method of claim 18, wherein the optical system and thehologram-bearing medium are arranged so that higher diffraction ordersof the hologram for the first viewing window have a zero point or anintensity minimum at the position of the second viewing window.
 20. Themethod according to claim 19, wherein the hologram-bearing medium isre-encoded for the second eye at the same time as the second viewingwindow is generated.
 21. The method of claim 1, wherein the holographicreconstruction is in color, and wherein the hologram-bearing medium iscomposed of cells arranged in a regular pattern with at least threeopenings per cell, representing the three primary colors, the phase orthe amplitude or the phase and the amplitude of light from the lightsource being controllable by said openings by encoding the hologram intothe hologram bearing medium, and said openings being encodedindividually for each primary color.
 22. The method of claim 1, whereina color reconstruction is achieved by at least three reconstructions inthe individual primary colors, generated sequentially.
 23. The method ofclaim 1, in which the hologram-bearing medium is a TFT display.
 24. Themethod of claim 1 in which the hologram-bearing medium controls phase oflight of the light source.
 25. The method of claim 1 in which thehologram-bearing medium controls amplitude of light of the light source.26. The method of claim 1 in which the hologram-bearing medium controlsphase and amplitude of light of the light source.